How to Walk on Water with Help from Dr. Seuss' Oobleck

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Lots of people have demonstrated that, surprisingly, if you fill
a pool with water and cornstarch you can run across it. Stop, and
you sink. How that happens, though, has been something of a
mystery in fluid dynamics.

The usual explanation for this " walking
on water " phenomenon: Suspensions — that's any liquid with
particles in it — are non-Newtonian fluids that get thicker, or
more viscous, as the rate of shear (deformation caused by, say,
running across it) goes up. Common examples are ketchup, blood
and toothpaste. "Normal" fluids, like water, flow and their
viscosity stays constant.

Scott Waitukaitis and Heinrich Jaeger, physicists at the
University of Chicago, have found that the situation is a bit
different: When you hit a suspension the particles get compressed
and transition into a solid state for a few moments.

Waitukaitis and Jaeger noted in their research paper, published
in tomorrow's (July 12) issue of the journal Nature, that the
usual mechanisms proposed weren't adequate to explain things like
how a person can run on the cornstarch-and-water mix (also known
as " oobleck "
in a homage to Dr.
Seuss and his book "Bartholomew and the Oobleck"). "Based on
a notion whereby the mechanism has to do with shear — where
sliding particles past each other generates an increase in
resistance to shear — that's a perfectly valid model, but it's
not enough to support a person's weight," Jaeger told
LiveScience. [ Twisted
Physics: 7 Mind-Blowing Findings ]

So the two scientists filled containers with oobleck and hit the
mixture with rods. Using high-speed photography and X-rays, they
saw that when the rod hits the suspension, there is a part of it
below the rod that becomes solid. The particles in the suspension
are jammed together, creating a columnlike region that is rigid
enough to keep the rod from sinking into the oobleck.

The thickened and now-solid region can propagate all the way to
the bottom of the container. Jaeger noted the solidifying zone
transmits force quite well — enough that at least one container
broke – and it can even rebound and push the rod back up if the
container is shallow enough. But it should be noted that contrary
to earlier models, the container walls — or lack thereof — don't
affect whether the rod or a running person stays on top of the
oobleck. The same thing would happen if you filled the ocean with
it and did the experiment again.

After the impact, the solid zone starts to melt away, since there
isn't any force compressing it anymore (except the small amount
due to gravity ). That's why when you stop running across the
oobleck you'd sink. It also means that while one can run on such
a substance, driving on it would be more of a problem — a wheel
isn't smacking down on a small region. [ See
Video of Oobleck Experiment ]

In fact, there is a minimum particle size for this phenomenon to
work; it's about one micron (or one one-millionth of a meter,
about the size of some bacteria). That's why milk, even though it
is
a suspension, doesn't behave like oobleck — the particles
just aren't big enough and in a high enough concentration.

The phenomenon is roughly similar to what happens with dry
particles. Jaeger and Waitukaitis got into this line of research
partly because of earlier work on making robots more flexible.
Particles seemed to be one answer — coffee bricks, for example,
are solid when compressed in their vacuum packaging by the
pressure of the surrounding air. Cut open the packet, though, and
the grounds pour out. This is the first time it's been
demonstrated in a liquid, though.

In addition to being a cool physics finding, Jaeger said there's
also a practical side. Some bulletproof vests take advantage of
this property, using silica particles suspended in polyethylene
glycol. By soaking Kevlar in the suspension, one creates a layer
of fluid that is held in place by the fibers. When it is hit, it
thickens, distributing the impact energy. That allows Kevlar
armor to be made with fewer layers, making it more flexible and
lighter.